For several years various solutions have been proposed to this technical problem. In this respect European Patent Application No. 0 225 667 which corresponds to commonly owned U.S. Pat. No. 4,803,990 describes a device of the kind set forth for measuring the velocity of moving organs and blood flows which utilizes the fact that the successive ultrasonic signals returned by a moving target are related, in the case of recurrent transmission with a recurrent period T, by the following equation: EQU S.sub.n+1 (t)=S.sub.n (t-.tau.) (1)
This means that the signal n+1 is the replica of the preceding signal n, except for a time shift .tau.. The latter represents the additional time required by the ultrasonic wave to travel the transducer-target-transducer path from one activation to another. In other words: EQU .tau.=2VT/C
where V is the velocity of the target and C is the velocity of sound. It appears that measurement of .tau. enables measurement of the desired velocity V.
The correlation function between S.sub.n (t) and S.sub.n+1 (t), defined by: ##EQU1##
The time to is linked to the scanning depth z as to =2z/C, and W is the integration window.
The function C.sub.nn (to,u) is an autocorrelation function and, therefore, it is maximum for u=o. Thus, a measurement of the time shift .tau., and hence of the velocity V, can be performed by searching the parameter u for which the function C.sub.n,n+1 (to,u) is maximum. To this end, the correlation function is sampled, using a sampling step .DELTA.t, between u.sub.min =-I.DELTA.t and u.sub.max =I.DELTA.t in steps of 1 so as to obtain 2I+1 correlation function values. The maximum value of these 2I+1 values corresponding to u=uo enables measurement of .tau. by utilizing the equality .tau.=uo.
In order to eliminate the errors which are inherent of the sampling during the determination of the maximum of the correlation function, use can be made of a multiplexing-interpolation circuit which supplies, on the basis of the correlation function values, a more exact estimate of the velocity and the value of the corresponding correlation peak. French Patent Application FR-2 590 790 in the name of Applicant gives the subject matter of which is included in the aforementioned U.S. Pat. No. 4,803,990 an example of this type of processing of the echographic signal where the correlation between signals is a "1-bit" correlation in a sense that the signals S.sub.n+1 and S.sub.n previously used are reduced to the sign of the ultrasonic signal. It is known that in that, case, the peak of the correlation function is shaped as an isosceles triangle. Knowledge of this shape enables complete reconstruction of the correlation peak by linear interpolation, starting from the highest point and its two neighbours, and hence exact determination of the position of uo.
This known method for the measurement of velocities, based on the analysis of the time shift, offers substantial advantages over other methods which are based, for example on frequency or phase shift. It notably allows for the use of troadband transmission signals, offering a high axial measurement resolution.
However, the method described above does not enable measurement of velocities exceeding a limit velocity V.sub.lim given by: ##EQU2## where C represents the propagation velocity of the ultrasonic wave. This phenomenon, also known as "aliasing", is linked to the indetermination induced by the periodicity of the echographic signal. A detailed description is given in "Doppler Ultrasound and Its Use in Clinical Measurement", P. Atkinson and J. P. Woodcock, Academic Press, 1982, pp. 114-128.
For example, using a recurrent period T of 100 .mu.s, a central acoustic frequency f.sub.o of 5 MHz, and a propagation velocity C of 1500 m/s, a limit velocity V.sub.lim of 75 cm/s occurs whereas, for example given blood flows can reach velocities which are substantially higher.
In order to increase the limit velocity of the measurement, it could be contemplated to decrease the frequency f.sub.o, but this would reduce the accuracy of measurement and the resolution. Similarly, an increase of the recurrent frequency would lead to an undesirable reduction of the scanning depth.
Thus, the technical problem to be solved by the present invention is to realise a device for the measurement of the velocity of blood flows of the kind disclosed in the preamble, which device enables an increase of the limit velocity V.sub.lim of measurement without reducing the frequency f.sub.o and without increasing the recurrent frequency 1/T.
Independent from the "aliasing" phenomenon, it is an object of the invention to remove other ambiguities which are linked to the sampling during the determination of the correlation peak. It may occur that the highest point of the sampled correlation function does not relate to the correlation peak searched. This situation may occur when complex streams are measured, comprising substantial speed gradients which tend to lower the correlation peak. This error becomes apparent as abrupt discontinuities in the reconstruction of the velocity profile as a function of the scanning depth.
A solution to the technical problem posed, known to Applicant, consists in that said device comprises a second processing channel for the echographic signal received, comprising
two symmetrical bandpass filters F.sub.1 and F.sub.2 which act on the signal S.sub.n (t), are connected in parallel and supply the signals s.sub.n1 (t) and s.sub.n2 (t), respectively, which are centred around a frequency f.sub.1 which is at the most equal to f.sub.o and around a frequency f.sub.2 which is at least equal to f.sub.0, respectively, the difference f.sub.2 -f.sub.1 being smaller than f.sub.0,
a multiplier which forms the product of the signals S.sub.n1 (t) and S.sub.n2 (t),
a symmetrical low-pass filter which selects the component S.sub.n (t) of the frequency f.sub.2 -f.sub.1 of the product S.sub.n1 (t) x S.sub.n2 (t),
a second correlation circuit which supplies 2I+1 sampled values of the correlation function of two successive signals S.sub.n (t) and S.sub.n+1 (t), referred to as the second correlation function, a multiplexer-interpolation circuit producing an estimate of the velocity by searching the maximum of the first correlation function around the sample producing the greatest value of the second correlation function.
Thus, the known device utilizes not only a signal S.sub.n (t) of high frequency (f.sub.0) like the prior art device, but also a second signal S.sub.n (t) of low frequency (f.sub.2 -f.sub.1) which also satisfies the relation (1) and which may thus be treated as the signal S.sub.n (t). The second correlation function, linked to the signal S.sub.n (t), has a frequency which is much lower and exhibits, in the measurement domain considered, substantially only one maximum, enabling total removal of the indetermination due to the "aliasing" phenomenon during the measurement of the velocity on the basis of the first correlation function. This device combines the advantages of a more exact measurement, determined by the signal of frequency f.sub.0, and a higher limit velocity, imposed by the low-frequency signal, and having a value which is given by: ##EQU3##
The known solution as disclosed in the foregoing paragraphs, however, does not allow for very high values to be obtained for the limit velocity. Actually, given the necessary separation of the filters F.sub.2 and F.sub.1 for selecting the frequencies f.sub.2 and f.sub.1, a ratio in the order of only from 2 to 3 can be obtained for the frequencies f.sub.0 /f.sub.2 -f.sub.1, enabling a gain from a value V.sub.lim of 0.8 m/s to a value in the order of 2 m/s. This is sufficient to measure all velocities of moving organs or flows in a healthy human body, but not for the measurement of given blood flows relating to pathological conditions, notably in the case of arterial stenosis, where the blood may reach velocities of 5 m/s, or arteriovenous shunts with blood velocities which are even higher.
Using the described known solution, when it is attempted to bring the frequencies of the filters F.sub.1 and F.sub.2 closer to one another, it is necessary to impart a much steeper cut-off edge thereto in order to prevent that they intersect one another, resulting in a parasitic DC component in their output signal. These filters then become more expensive because they are longer, which also implies the drawback of a loss of resolution and less exact correlations.